High voltage switchgear, as defined herein, consists of a metallic enclosure containing a plurality of industrial-rated high voltage circuit breakers capable of interrupting currents at applied voltages in excess of 1,000 volts. When the switchgear is de-energized, for inspecting and repairing associated industrial equipment, is is required by state and national electric codes that the switchgear terminal bus be connected directly with ground. In the event that the switchgear is inadvertently energized while such inspection and repair is ongoing, this would prevent serious damage to the personnel, as well as to the associated equipment.
Electrical connection with the high voltage switchgear terminal bus is made by employing a "hotstick" which generally includes a long insulated handle with means for mechanically manipulating an insulated hookshaped end. When the terminal lugs are attached to the high voltage switchgear terminal bus, an insulating cylindrical body terminating in an eyelet is attached over the terminal lug to prevent localized corona effects. A separate ground stud is bolted to the high voltage switchgear terminal bus to facilitate connecting the terminal bus to ground when the switchgear is de-energized. The ground connector generally comprises a spring-loaded clamp at one end of a heavy gauge metal conductor that is bolted to the system's ground terminal at an opposite end. An insulative cylinder, terminating in an eyelet, is arranged over the ground stud in the same manner as for the high voltage lugs. The threaded part of the ground stud that extends through the terminal bus is wrapped with electric insulating tape to prevent localized corona from otherwise occurring with exposed metal parts at voltages in excess of 1000 volts. To access the ground stud, the hooked end of the hotstick is inserted through the eyelet at the end of the insulative enclosure to unscrew the enclosure from the protected ground stud. Using the same hotstick, the ground conductor is attached to the exposed ground stud by means of the spring-loaded ground clamp. Before energizing the high voltage switchgear, the procedure is reversed and the ground connector clamp is removed. The insulative enclosure is positioned over the exposed ground stud and is threaded onto the exposed ground stud threads.
To assist in first attaching the ground stud to the switchgear terminals, a hex-shaped part is formed on the ground stud for receiving a wrench or similar tool. The The threads formed on the ground stud for engaging the switchgear bus conductor terminal is formed on the ground stud next to the hex-shaped part and the opposite side of the threads is turned to a diameter less than the threaded part to accommodate receiving the grounding clamp. Over long periods of continued use, the grounding clamp contacts the threaded part and eventually causes some of the threads to become impaired. When the insulative enclosure is next attached to the damaged stud threads, some portion of the ground stud accordingly remains exposed to the atmosphere. Localized corona arcing then occurs, causing pitting and erosion of the insulated enclosure until the ground stud and insulative enclosure must both be replaced. The arrangement of the electrical insulating tape around the threaded part of the ground lug that engages the high voltage switchgear terminal bus, must be cut away in order to allow the ground stud to be removed and replaced. Further, in damp locations, moisture can eventually penetrate through the taped part of the ground stud, which then requires removal and replacement of the insulating tape.
One purpose of the instant invention is to provide a ground stud arrangement that readily facilitates connection and removal of the ground stud from the high voltage switchgear terminal bus without damaging the ground lug threads and without causing any localized corona formation whatsoever.